Internal combustion engines need to have adequate compression levels within each cylinder in order to operate properly. Accordingly, compression tests may be routinely performed to diagnose the compression level in each cylinder. Inadequate cylinder compression may be used to diagnose cylinder components such as valves, gaskets, rings, etc. Typically, compression tests may be intrusive requiring the disassembly of the engine. For example, spark plugs and/or fuel injectors may be removed from the engine by a service technician so that a pressure transducer can be inserted to measure individual cylinder pressure levels. Furthermore, the engine may need to be rotated unfueled (e.g., via a starter motor) to a desired position in the engine cycle. However, such intrusive approaches tend to be difficult and time consuming, and also require a trained technician. Further still, the removal of engine components (e.g., spark plugs) to perform the test renders the removed components prone to damage.
One example approach for performing a non-intrusive compression test is shown by Schroeder et al in U.S. Pat. No. 4,539,841. Therein, a probe in the form of an electromagnetic speed sensor is used to measure instantaneous engine speed over the compression phase of each cylinder during engine idle conditions. The estimated engine speed is used to infer cylinder peak pressure and peak power outputs. By comparing the inferred values to a normalized pressure curve, cylinder compression issues are diagnosed. Another example non-intrusive compression test is shown by Gerbert et al in U.S. Pat. No. 5,663,493. Therein, a service tool is used to monitor the battery voltage or current in an engine during the cranking phase where the engine is rotated using a starter motor which draws current from the battery. The tool displays the relative compression of each cylinder on a screen to the technician based on the variation in starter current.
The inventors herein have recognized potential issues with such approaches. As one example, while the approaches are non-intrusive, they nonetheless require a technician for completion. Specifically, both approaches require the use of diagnostic tools that have to be attached to specific locations of the engine or vehicle by a trained technician. In the approach of '841, the use of an idling engine may also add substantial noise factors that can corrupt the diagnostic results. In the approach of '493, the method is limited to assessing changes in battery current at engine cranking speeds. As such, more significant current changes may be available at other higher engine speeds. In addition, the limited air flow at the cranking speed can add substantial noise factors. In either case, the diagnostics may be time consuming and the results may be error prone (e.g., prone to human errors or noise-induced errors).
In one example, some of the above issues may be at least partly addressed by a method for a hybrid vehicle system comprising: spinning an engine unfueled using motor torque with intake valve timing advanced and an intake throttle open; and based on a change in battery current during the spinning, indicating cylinder compression degradation. In this way, a compression test can be performed automatically and non-intrusively without requiring a trained technician to disassemble engine parts or use complex diagnostic tools.
For example, while a hybrid electric vehicle system is in a key-off condition, a cylinder compression test request may be received at the vehicle from a service technician or operator. The request may be made via interactions with a touch-interactive display of the vehicle, or via a simple diagnostic tool. In response to the compression test request, the controller may confirm that the engine temperature is within a defined range (e.g., above 60° F.). Upon confirmation, the engine may be spun unfueled using motor torque from a battery-driven electric motor/generator used to propel the vehicle. The engine may be initially spun at a lower speed (e.g., 1000 rpm) for a shorter duration (e.g., 15 seconds) to reduce friction variability in cylinders from lack of oil film and to build sufficient pressure at an engine-driven mechanical oil pump to help actuate engine cams. The engine may then be spun at a higher speed (e.g., 1000 rpm-2000 rpm or more) for a longer duration with intake valve timing fully advanced (via intake cam timing advance) and with an intake throttle fully open. The engine speed may be adjusted to be the lowest achievable engine speed that allows engine speed control to be retained while driving manifold pressure (MAP) towards barometric pressure (BP). By opening the throttle and advancing the intake cam, more air may be ingested in the engine cylinders enabling higher peak pressures to be attained. As such, this significantly improves the signal-to-noise ratio.
A change in the battery current (or voltage) is then monitored over one or more engine cycles, specifically, during the compression stroke of each cylinder. Since the battery has internal resistance, the voltage across the battery terminals varies as a function of engine spinning resistance, which varies according to the compression in each cylinder. The change in current is used to estimate the torque produced in each cylinder's compression stroke. The estimated torque is then used to infer each cylinder's compression pressure using scalar algorithms. By comparing the inferred compression pressure of each cylinder with an expected cylinder compression pressure, degradation of cylinder compression can be identified and a diagnostic code can be set identifying the specific cylinder(s) that failed the test. A service technician can then diagnose the affected cylinder.
In this way, a cylinder compression test can be performed non-intrusively without requiring engine disassembly. By enabling the test to run automatically following a test request by a service technician, substantial time and cost reduction benefits are achieved. In addition, damage to spark plugs threads and other engine components may be reduced. By inferring the cylinder pressure based on the change in battery current, the need for dedicated cylinder pressure transducers is reduced. Overall, a simple yet accurate non-intrusive compression test is provided.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.